The invention relates to a micromechanical sensor integrated on a chip, including a semiconductor substrate, an electronic circuit, a void, a diaphragm, a counterelectrode and valve openings connecting a volume of the void to its surroundings.
Highly miniaturized sensors are desired for certain applications, such as pressure sensors or microphones. Pressure differences or pressure fluctuations or acoustic oscillations can, for instance, be measured with the aid of the sensors.
A micromechanical sensor for applications and acoustics is described in U.S. Pat. No. 5,146,435. The sensor described therein includes a movable diaphragm and a supporting structure formed of a silicon wafer. A counterelectrode disposed above the web wafer protrudes past a wafer surface and forms a void between the diaphragm and the counterelectrode. The counterelectrode has valve openings on its upper surface, which communicate with its surroundings for the sake of air exchange with the void. The silicon wafer is removed below the movable diaphragm, so that the acoustical signal can reach the diaphragm. The principal by which the micromechanical sensor operates is based on a change in volume of the void, which causes a change in capacitance of the capacitor formed by the diaphragm and the counterelectrode. Electrical contacts are located on the upper surface of the wafer in order to measure the capacitance. No evaluation electronics are provided on the wafer.
German Patent DE 196 48 424 C1 describes a micromechanical sensor which can be used for pressure measurements and acoustic measurements. The sensor is constructed on an SOI (silicon oxide isolator) substrate and includes a diaphragm of polycrystalline silicon, which is disposed above a spacer layer on the substrate underlay. The spacer layer and the polycrystalline diaphragm form a void, and a counterelectrode is located in the region of the substrate on the opposite side of the diaphragm. The counterelectrode is thus disposed below the diaphragm. The diaphragm is located in the direction of the upper surface of the chip. In order to provide air equalization, valve openings are present which are aimed in the direction of the lower surface of the substrate. The polycrystalline diaphragm together with the other levels forms a chip surface that is not plane.
One disadvantage of the above-described structure for a micromechanical sensor is that the sensor cannot be produced in a simple way. Due to the diaphragm surface disposed at a distance from the substrate surface, the topology of the semifinished sensor during production is not suitable for applying a semiconductor circuit to it in a simple way. Manufacturing the circuit with a small number of process steps is therefore impossible.
Another disadvantage of the above-described configuration is that the diaphragm is formed of polycrystalline silicon. Polycrystalline silicon has unsatisfactory properties with regard to mechanical strength, which involves problems in manufacture, for instance. Thus the semifinished sensor element must be manipulated substantially more carefully, if damage to the vulnerable sensor surface is to be avoided. The production of known micromechanical sensors with a diaphragm oriented in the direction of the upper surface also has disadvantages. For instance, sacrifices in product quality can come about in mounting or when the wafer is sawn or diced apart, for instance.
It is accordingly an object of the invention to provide a micromechanical sensor and a method for producing the same, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a micromechanical sensor integrated on a chip having a chip surface, the sensor comprising a semiconductor wafer substrate having an upper surface; a void having a volume; a diaphragm; valve openings directed toward the upper surface of the wafer substrate and connecting the volume of the void to the surroundings; a counterelectrode forming part of a coating plane extending entirely over the chip surface, so that an electronic circuit can be applied to the coating plane by known semiconductor technology.
In accordance with another feature of the invention, the membrane is formed of monocrystalline material.
In accordance with a further feature of the invention, the counterelectrode is conductive or has a conductive layer.
With the objects of the invention in view there is also provided a method for producing a micromechanical sensor, which comprises a) producing a sacrificial layer in one region of a substrate surface of a semiconductor wafer substrate having a rear side; b) epitaxially growing a semiconducting material layer to create a planar layer protruding past the vicinity the sacrificial layer, and growing the semiconducting material layer in polycrystalline form in the vicinity of the sacrificial layer; c) optionally smoothing the planar layer; d) etching openings and filling the openings with a suitable material; and e) removing the material from the openings and the sacrificial layer, and etching the rear side of the wafer substrate for laying bare a diaphragm and creating a sensor opening.
In accordance with another mode of the invention, there is provided a method which comprises providing the wafer substrate with an oxide intermediate layer acting as an etching stop in the step of etching the rear side.
In accordance with a further mode of the invention, there is provided a method which comprises stopping the rear side etching by suitable doping of the wafer substrate.
In accordance with an added mode of the invention, there is provided a method which comprises applying a seed layer to the sacrificial layer after step a), the seed layer assuring improved growth conditions in the growth of the planar layer in step b).
In accordance with an additional mode of the invention, there is provided a method which comprises wet etching the rear side etching non-electrically or electrochemically, depending on the layer used for the etching stop.
In accordance with yet another mode of the invention, there is provided a method wherein the sacrificial layer and the material filling the openings are silicon oxide.
In accordance with yet a further mode of the invention, there is provided a method which comprises producing an electronic semiconductor circuit in a known manner after step d) and before step e).
With the objects of the invention in view there is additionally provided a microphone or pressure sensor having the micromechanical sensor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a micromechanical sensor and a method for producing the same, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.